The technical state of the art comprises stepping diggers as disclosed in the Swiss patent document 508,786 which are designed to move on terrains with extreme slopes and other irregularities. Their chassis comprise height and side adjustable wheel arms with wheels and further height and side adjustable support legs with resting claws. A superstructure pivotable about a vertical axis and mounted on the chassis supports the driver's cabin, the drive units and digging arm with digging buckets. By means of these and the two wheels, the vehicle when stepping rests on the ground while the support legs are lifted off the ground. By stretching and bending the digging arm, the stepping digger can be moved on its wheels. Once the digger has reached its final position, the chassis is deposited by its support legs on the ground and thereby the digger arm can initiate a new step.
In order to steer the stepping digger, the superstructure is constrained to rotate while the chassis is lifted on one side until the desired direction of travel has been reached. This procedure can be implemented only in small steps of a few degrees because the moment the tripod consisting of the two wheels and the digger arm has been altered to a point that the vehicle's center of gravity wanders outside the triangle, the vehicle shall tip over a claw or totally.
As soon as the bucket at the digger arm has been replaced by a freely hanging clamshell, the stepping process no longer is feasible because the clamshell does not transmit longitudinal or transverse forces. Other implements as well, for instance rock drills or wood reapers at the digger arm, do not permit stepping motions.
The U.S. Pat. No. 4,779,691 discloses a terrain vehicle which comprises a chassis similar to that of a stepping digger but lacks a digger arm pushing or pulling the vehicle, instead the two above described support legs are designed to be stepping legs which alternatingly support the vehicle on the ground by being the third resting point or are lifted off the ground. In the continuous motion with alternating use of the legs, a tripod is formed as for the stepping digger, said tripod being formed of two wheels and one leg. The lateral triangle lengths vary with each change of step and with each steering procedure. Accordingly the stability (reliability against tipping over) of the vehicle changes continuously and frequently is restricted to a narrow residual triangle, so that in the case of adverse loading and/or on a slope there will be danger of tipping over.